Giving a treat following a behavior or series of behaviors, the dog may be unsure what exactly the reward was for...unless its Lassie or another dog who understands English.
Think of it this way.
If you were teaching someone a series of behaviors, like martial arts,
it would look like this: "Show me a front kick" (command), "yes that
was the correct move"(click), "good job" (reward)." The "click" does that for you. It also allows you to chain small behaviors together to form a more complex behavior.
For example. Front snap kick. Bring leg up (click)...Bring leg up
(click)...Bring leg up (no click)..bring leg up..extend leg (click).
Think of the click as a way of explaining to an animal "that is what I
want." Sure, the treat relays the same message but the click just makes it more percise. Its how trainers get dolphins to do flips and spins on command

You say sit, and then click when the dog's butt touches the ground to mark the exact second when it was in the right position.

Now, if you treat without the click, you say "sit" the dog's butt touches the ground and then you go to give him the treat and he looks to the left. Now, did you treat for looking to the left or sitting?

That may sound like a dumb example, but I taught Ruby to spin in circles by clicking when she looked left, then when she turned a little left, etc etc etc. Before long she was spinning in circles with no problem.

when I was not timing the click correctly I was sending her all sorts of bad clues. for quite a while, she thought "spin" meant "touch the bike pedal with your nose"

Michelle

Inside me is a thin woman trying to get out. I usually shut the bitch up with a martini.

For a while, when I was leash training Molly, I would do the STOP the minute they are pulling and wait until the leash goes slack, than walk again. Molly usually stopped to look back at me wondering why we weren't going anywhere and we'd continue on...somehow she learned (I'm sure poor timing on my part) that if we stopped, she was supposed to turn in a circle at the end of the leash. I mean, the leash went slack and she stopped pulling, but I didn't exactly intend for the spins.

A clicker probably would have been very helpful in this instance.

Or maybe she was spinning in circles because she was confused on her pack order. (Sorry...OT...I've been reading waaaay to much dominance-is-the-core-of-all-"badness" junk lately)